1. Field of the Invention
The present invention relates to an ion current detector to detect the ionization of combustion gas generated by the combustion of an internal combustion engine as an ion current, and thus to detect a combustion status on the basis of the ion current.
2. Description of Related Art
An ignition-type internal combustion engine (hereinafter referred to as engine) compresses a mixed gas of air and fuel for introduction into a combustion chamber (hereinafter referred to as cylinder) by a piston and ignites and burns the compressed gas with an ignition plug to produce power. An output from an engine and ignition timing for a cylinder position share a close relationship. Generally, when the ignition timing is advanced, higher output is attainable. However, in the presence of an excess advance angle, an abnormal combustion condition called knocking takes place. In the case of repeated knocking, the engine can be destroyed.
When the mixed gas burns inside the cylinder, the burning gas is ionized. Therefore, when a voltage is applied to the burning gas, a current flows within the burning gas due to the ions' action. This current is called an ion current, and it varies according to the burning condition inside the cylinder. Accordingly, the burning condition can be determined by detecting the ion current.
The ion current sharply increases immediately after ignition, and reaches its peak in a short period of time, and thereafter gradually decreases. In the case where knocking occurs, an oscillating component of several kilohertz is superposed onto the ion current. It is indispensable to extract only the oscillating component caused by knocking from the sharply varying ion current. In Japanese Patent Laid-Open No. 7-63869, an ion current-to-voltage conversion part comprising an ion current detecting part to detect an ion current, and a diode, has been proposed. This ion current-to-voltage conversion part outputs a voltage in proportion to the ratio of an alternating component (oscillating component) included in the ion current which has been detected in the ion current detecting part by using logarithmic characteristics of a forward voltage of the diode.